The electronics industry is striving to further miniaturize semiconductors and integrated circuits while increasing design complexity. To accomplish that result, the individual active electrical devices such as transistors, diodes and the like used to create the circuitry and the interconnects between such devices must be fabricated on an increasingly small scale. As circuitry dimensions decrease, contaminants present during the fabrication of electronic devices cause significantly more device failures and malfunctions. Therefore, surface contaminants must be carefully removed to maintain quality standards and to maximize the yield of fully functioning integrated circuits.
Contaminants present during integrated circuit fabrication include photoresist materials, residual organic and residual metallic contaminants such as alkali metals and native/metallic oxides. Metallic films comprising metal oxides and metal halides are also inadvertently deposited onto electronic devices during immersion into etchant or resist stripper baths, both which may contain metal ions and free metals in solution. Likewise, corrosive chlorides are deposited on such assemblies contaminants can weaken or embrittle the electrical connectors of the device as well as delaminate the layers of the device resulting in current leakages or physical failure.
Chemical cleaning agents are typically used to remove bonded or adsorbed metallic oxides and corrosive chloride residues from wafer surfaces between the numerous individual steps required to fabricate an integrated circuit. Conventional chemical cleaning is typically performed using a series of acid and rinse baths. These cleaning methods are often characterized as "wet" techniques because the electronic device is immersed in a liquid cleaning solution immediately prior to performing the next processing step. Numerous problems are encountered when using liquid cleaning solutions including incomplete removal of the cleaning agent from the surface, introducing new contaminants onto the surface to be cleaned as well as disposing of hazardous liquid wastes.
A typical wet cleaning process for removing film contaminants from a bare silicon or thermally grown silicon oxide crystal comprises the steps of:
(1) immersing the wafer in an inorganic resist stripper such as sulfuric acid/hydrogen peroxide followed by immersing in a sulfuric acid/oxidant mixture and rinsing with deionized water;
(2) immersing the wafer into a mixture of water/ammonium hydroxide/hydrogen peroxide to remove metal oxides and metals followed by rinsing with deionized water;
(3) immersing the still wet wafers into a mixture of water/hydrochloric acid/hydrogen peroxide to desorb atomic and ionic contaminants; and
(4) rinsing with distilled water and drying in a inert atmosphere such as nitrogen. (4) rinsing with distilled water and drying in an inert atmosphere such as nitrogen.
The above mentioned "wet" cleaning process suffers from numerous problems: First, ammonia and HCl vapors can mix to form particulate smoke containing colloidal ammonium chloride particles which can cause wafer contamination. Special care must also be taken to prevent depletion of hydrogen peroxide from the cleaning solution of step (2) because ammonium hydroxide in the absence of hydrogen peroxide acts as a silicon etchant. Additional contaminants may also be introduced into the system during the numerous distilled water rinsing steps required to remove cleaning residue. Finally, trace moisture must be removed, typically via high temperature vacuum applications, before conducting the next processing step.
Disadvantages associated with wet wafer cleaning processes have led to a search for "dry" cleaning processes wherein the cleaning agent is applied and removed in the vapor state. In order to conduct a vapor phase cleaning process, the products formed by contacting the cleaning agent and the contaminants must possess sufficient volatility to enable essentially complete removal from the surface to be cleaned. Manufacturers are continually searching for "dry" cleaning agents and vapor-phase processes for using such cleaning agents which eliminate the enumerated problems wherein high quality electronic devices can be fabricated without using environmentally harmful reagents.